Tim Mitchell wrote:

Sounds to me like the sealant you've used is acrylic or partly
acrylic rather than silicone. You need to use "sanitary grade
silicone", you will recognise it because it's the most expensive
stuff on the rack! It
is very flexible and should take up a lot of movement. Stinks of
vinegar while curing.

The unibond one I used says it is water based acrylic
kitchen/bathroom sealant. Says it is waterproof when dry. And shows
a picture of someone applying it around a bath on the tube.....

Well, all sealants are waterproof, but all are not created equal. If
you use the expensive silicone you will not have the problem. Some of
the cheaper ones are OK too but it's such a pain scraping it off, you
may as well spend a few quid more and use the best stuff. Your
choice...

Well, yes, but the expensive silicone is what I thought I was buying!